While many observables in heavy-ion collision scale with the number of nucleons participating in the collision, hard processes do not trivially scale with the the number of hard binary collisions due to shadowing effects [1]}.
nevertheless, the number of participating nucleons is a key quantity to classify heavy-ion reactions at high energies.
Centrality allows a good classification of the transverse energy production, which in turn is a good measure of the entropy of the system.
As the centrality increases, more and more nucleons take part in the formation of the nuclear fireball.
In experiment, the number of charged particles in the acceptance of the detector system is usually used to approximate the centrality.
Glauber Monte Carlo simulations can be used to relate a certain multiplicity class to the respective centrality class defined by a mean number of participating (primordial) nucleons \(\langle A_{part}\rangle \)  [2]}.
Its has been shown, that this strategy can be applied for experiments with beams of a few  \(\,A\)  G\(e\) V on stationary targets [3]} up to a few  \(\,A\)  T\(e\) V energies at the LHC [4]}.
At ultra-relativistic energies the initial stage of the collision does not contribute substantially to the yield of soft (virtual) photons.
However, the high pressure in the initial state and the flow generated out of it has some effect on the momentum distribution of the (virtual) photons (cf. sec. REF ).
